Your questions answered: Space elevators

To high heaven: As space elevators move from the realm of science fiction to reality, our panel of experts answers your questions

Space elevators have been mainstays of science fiction for decades. Arthur C Clarke was particularly fond of the concept, using it in no fewer than five of his novels; other luminaries such as Kim Stanley Robinson, Robert Heinlein and Iain M Banks also used the idea of a tower with its top in a geostationary orbit, allowing payloads to be sent into orbit by hauling them up the tower rather than using chemical rockets to get them there. The idea has a long history, being first proposed long before the first real rockets, in 1895, by the Russian space science and astronautics pioneer Konstantin Tsiolkovsky; but it has gained more currency in recent decades because it’s thought that it might be a cheaper way to attain orbit. The practical considerations, such as materials science and the actual mechanics of the elevator system, have been seen as so onerous that it could only be a concept.

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NASA concept for the lift portion of a space elevator

But a Canadian company, Thoth Technologies, has been awarded a patent for a partial space elevator concept, with a tower 20km high topped with a runway for space plane-type launch systems to take payloads the rest of the way up to orbit. We sent readers’ questions to Thoth, whose chief executive, Caroline Roberts, and the inventor of the system, Brendan Quine, Thoth’s chief technology officer, a professor at the Lassonde School of Engineering at York University in Toronto, have provided answers (TT).

The president of the International Space Elevator Consortium, Peter Swan (PAS) has provided answers to questions that were more concerned with the general concept of space elevators than the Thoth concept. Dr Swan is a senior scholar at the NRO Centre for the Study of National Reconnaissance, a retired lieutenant colonel in the USAF and has taught at Delft University and the Stevens Institute of Technology.

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The Thoth space elevator concept

What is the Thoth structure made from, what is it inflated with, and what is its aspect ratio?

TT: The ThothX Tower is made from readily available materials, such as Kevlar and polyethylene, inflated with helium or hydrogen. The patent example structure describes an aspect ratio of 50:1.

As 90 per cent of the energy required to enter orbit is kinetic, a space elevator would knock around 10 per cent off the potential energy, so less than 1 per cent off the orbital needs. How can less than 1 per cent off energy lead to 30 per cent off cost?

TT: As we describe in A free-standing space elevator structure: a practical alternative to the space tether BM Quine, RK Seth, ZH Zhu Acta Astronautica 65 (3), 365-375, 2009, rockets consume approximately 39 per cent of their fuel during the initial ascent phase to 20km. The reduction in fuel usage comes with a corresponding benefit in the number of stages needed to reach orbit (only one stage is required for a launch at 20km versus three or four for conventional launch). The 1 per cent energy estimate claim does not take into account the staging aspect of rocketry (the rocket is extremely heavy with stages and fuel
at launch and very light by orbit). Rocketry is extremely energy inefficient with only about 3 per cent of the chemical energy going into raising to payload to orbit. Thus massive amounts of fuel and hardware must be raised initially to have enough left to propel the final injection stage. Electrical elevators are 50 to 60 per cent efficient leading to a significant fuel saving advantage that enables single stage to orbit space planes to fly from the top of the tower. These planes can also be completely reusable such as a passenger jet as opposed to being single use such as current rockets. This reaps a very significant hardware cost advantage that will reduce the cost of space access.

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The top of the space elevator structure is a runway for spaceplane-type launch vehicles

Could an electromagnetic launcher be run up the inside of the tower?

TT: Magnetic launch systems hold great promise to launch small compact payloads and might be best deployed at the top of the tower where satellites could be launched directly into orbit.

For a true space elevator, i.e. with the top in orbit, would construction be from the ground up or from the orbital end down?

PAS: Two points that are important to remember. There is only one point on the 100,000km space elevator that is ‘in orbit’ and that is at the GEO location. You are only ‘in orbit’ if you release from the space elevator, which is a permanent attachment to the Earth and rotates with it. When you release from the SE, you go into some type of Keplerian motion – below approximately 24,000km, the ellipse does not miss the Earth on the far side with its perigee. Between 24,000km and GEO the ellipse has apogee at release point and perigee below that. At GEO, approximately 36,000km altitude, you release into a circular orbit maintaining the relationship with the Earth, in the same way as GEO satellites. Beyond GEO, you go into ellipses that have the apogee well above the GEO orbit – at 47,000km altitude release you go into an orbit that has apogee near the Moon, with a 57,000km release point enabling you to go beyond Earth’s pull and reach Mars. At the end of the space elevator (roughly 100,000km), a release will enable you to reach the outer planets and then go beyond the solar system with planetary assists. All this motion is due to the kinetic and potential energy provided by the motion and height of the release location. No chemistry is required to start your journey. Rockets are then needed to adapt your orbit, provide trajectory changes or land on large bodies. Second, The current concept for construction of a 100,000 km space elevator is the following:

Step one: lift cable to LEO, raise it to GEO (with the old-fashioned method, rockets).

Step two: lower the ‘starter’ tether from GEO to the surface of the Earth. This initial tether will just be robust enough to support itself and small climbers. 

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The base of the elevator could be located at sea and used as a port

Step three: Build up the tether with use of small climbers.
These small climbers would leave the surface of the Earth with additional tether to be combined to the starter tether such that the tether would grow in capability.

TT: The ThothX tower is built from the ground up. Thoth is also pursuing patents for construction techniques, including an extrusion method. We believe the Japanese are planning a space tether that uses a counter-balance beyond geostationary orbit to hold up the mass of the tether. This likely requires in-space construction (top down). The main challenges of this design are materials strength (carbon nano-tubes are not strong enough) and lightning strikes that could sever the tether. If a tether is to be realised then it would be advantageous to mount it on the top of a 20km pneumatic tower to avoid the environment of the lower atmosphere.

 Would there be any advantage in using balloons to lift payloads to a platform, using the barge principle of moving large weights slowly rather than small weights fast?

TT: We believe balloons have limited utility because of their lift capacity. The inventor has experience in launching heavy-lift balloons to the stratosphere, however, the payload is limited to a few tonnes. A similar effect could be achieved using an evacuated shaft and gas pressure to raise payload.
It is also highly likely that a regenerative elevator design would be used to harness the potential energy released during descent.

What measures would be taken at design stage to prevent buckling, and what systems would be in place to ensure the integrity of the structure once built?

TT: The problem of structural wrinkling (the onset to buckling) has been addressed by previous research (see ‘Experimental investigation of inflatable cylindrical cantilevered beams’ ZH Zhu, RK Seth, BM Quine, S Okubo, K Fukui,
Q Yang, T Ochi, JP Journal of Solids and Structures 2 (2), 95-110, 2008). The core is not comprised of a single gas cell the diameter of the structure, but by many cells arranged in a torus. Consequently, maintenance can occur to repair leaky cells without compromising integrity. The research paper lays out experimentally derived guidelines for pneumatic structures to avoid the onset of wrinkling, which we have adopted in our design. The control system is implemented in a multiply redundant computer system much like that equipping a modern jet where at least three systems are used in hot redundancy so as
to ensure continuous operation. Using external sensor data such as wind velocity and building attitude, the control system would command control responses from actuators that would adjust the structure’s stance in order to counter external forces in a harmonic control strategy that is described in the patent.

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A climber ascends a tether-type space elevator

 What’s the magnitude of the forces that would act on a structure such as this owing to air pressure, and to what height would they act? Are there any forces resulting from the rotation of the Earth? 

TT: The structure is designed to withstand a Category 5 hurricane with wind speed of 156mph with significant safety margin and so the sheer and turbulent forces of a thunder storm are within this design envelope. The structure’s pressurisation would exceed 100 times atmospheric pressure so pressure changes would have limited effect. The large structural mass is leveraged by the design to negate external forcing by steering the structure’s centre of gravity. There are forces resulting for Earth rotation but we have estimated these as quite small compared with other forcing factors. There are also thermal stress effects to be countered.

 The ISS regularly has to adjust its position to avoid space debris. At what altitude would this become an issue for a space elevator? And how could it be designed to deal with this problem?

PAS: For the full space elevator, the answer is two fold. For small stuff, that is not tracked, accept the hits and design the tether for it, with repair climbers going up or down once a year. For the 200 to 2,000km altitude region of the space elevator, the tether would be designed to accommodate this threat. The tether would probably be 1m wide, woven to spread the tension and adaptable for any strand penetrations. In addition, the tether would be curved so no single small piece of debris could cut the tether.  For trackable debris, the tether would be moved. This requires excellent tracking of space debris (expected by 2030 or so) and the ability to move the tether (there are many ways o do this: reel in, out, move tether climbers up/down faster slower, actually use thrusters at the GEO Node or Apex Anchor, or actually move the Marine Node). The knowledge of each segment of the tether can be simulated and updated with measurements in real time to ensure the knowledge is available for predictive avoidance. The calculations of the threat to space elevators – hits and density of threat – are in the report: Space Elevator Survivability, Space Debris Mitigation 2011, The bottom line on space debris and space elevators is that it is an operational issue that can be successfully managed with design and procedures.

 TT: For a 20km tower, space debris would not be a significant problem as most meteorites burn up before reaching 20km. Like the ISS, the ThothX tower can adjust its position. The main novelty of our patent is in the harmonic control strategy that we describe that continuously monitors and corrects the structures stance in order to control it. The centre of gravity is guided actively over the base in order to null out external forces such as a hurricane. We also anticipate that structural maintenance and repair would be necessary as with any real construction.